Structural Study and Detrital Zircon Provenance Analysis of the Cycladic Blueschist Unit Rocks from Iraklia Island: From the Paleozoic Basement Unroofing to the Cenozoic Exhumation

Detailed mapping and structural observations on the Cycladic Blueschist Unit (CBU) on Iraklia Island integrated with detrital zircon (DZ) U-Pb ages elucidate the Mesozoic pre-subduction and the Cenozoic orogenic evolution. Iraklia tectonostratigraphy includes a heterogeneous Lower Schist Fm., juxtaposed against a Marble Fm. and an overlying Upper Schist Fm. The contact is an extensional ductile-to-brittle-ductile, top-to-N shear zone, kinematically associated with the Oligo-Miocene exhumation. The DZ spectra of the Lower Schist have Gondwanan/peri-Gondwanan provenance signatures and point to Late Triassic Maximum Depositional Ages (MDAs). A quartz-rich schist lens yielded Precambrian DZ ages exclusively and is interpreted as part of the pre-Variscan metasedimentary Cycladic Basement, equivalent to schists of the Ios Island core. The Upper Schist represents a distinctly different stratigraphic package with late Cretaceous MDAs and dominance of Late Paleozoic DZ ages, suggestive of a more internal Pelagonian source. The contrast in the DZ U-Pb record between Lower and Upper Schist likely reflects the difference between a Paleotethyan and Neotethyan geodynamic imprint. The Triassic DZ input from eroded volcanic material is related to the final Paleotethys closure and Pindos/CBU rift basin opening, while late Cretaceous metamorphic/magmatic zircons and ~48–56 Ma zircon rims constrain the onset of Neotethyan convergence and high-pressure subduction metamorphism.

Lithospheric-scale extension during Grampian orogenesis in Scotland

This contribution presents a new model for the Grampian-age tectonothermal development of the Buchan Block and Barrovian-type regions to its west, in the Grampian Terrane, Scotland. The model has drawn on evidence gathered from field mapping, microstructural analysis, metamorphic petrology and mafic magma geochemistry to propose that emplacement of the Grampian gabbros and regional metamorphic heating associated with production of Barrovian- and Buchan-type units occurred during syn-orogenic (Grampian-age), lithospheric-scale extension. Extension followed lithospheric thickening associated with the initiation of Grampian orogenesis and was followed by renewed lithospheric thickening and termination of the extensional heating. Mantle melting to produce the Grampian gabbros of the Grampian Terrane was achieved by extensional thinning of the lithosphere and decompression melting of the asthenosphere at depths of less than 70 km. Advection of heat from the mantle with emplacement of the Grampian gabbros augmented elevated heat budgets associated with attenuation of isotherms during extension. Deposition of the uppermost Dalradian (the Whitehills and Boyndie Bay Groups and the Macduff Slates) occurred during Grampian-age lithospheric extension. A gently-dipping, mid-crustal detachment focused metamorphic heat sources and accommodated significant lithospheric-scale strain, allowing independent thermal evolution of units in its hanging wall (the Buchan Block) and footwall (Barrovian-type units).